Optical coherence tomography (OCT) is a noninvasive optical imaging technique which provides cross-sectional or three-dimensional images samples with an axial resolution of 1-15 μm. One of the main applications of OCT is in ophthalmology. Amongst the various known OCT systems, Fourier domain OCT (FD-OCT) is significantly faster than time domain OCT (TD-OCT) and has an improved signal to noise ratio (SNR).
But, FD-OCT suffers from an inherent sample-independent limited depth range, typically between 1 and 5 mm. One limitation flows from the fact that FD-OCT extracts depth information from the inverse Fourier transform of a spectral interferogram. Since the spectral interferogram can only be recorded as a real signal, its Fourier transform is necessarily Hermitian symmetric about the zero path length difference (ZPD) position. As a result, the positive and negative displacements about the ZPD cannot be unambiguously resolved, giving rise to mirror image artifacts and generally halving the useable range. This is referred to as the complex conjugate ambiguity. Another limitation is a sensitivity fall-off, which results in reduced sensitivity with increasing depth. Moreover, since the OCT's signal is derived only from backscattered photons, optical attenuation from absorption and scattering generally result in a useable imaging depth of less than 2 mm.
Several “full range” OCT techniques have been developed that eliminate the complex conjugate artifacts to effectively double the measurement range around the ZPD position. Recently, a Dispersion Encoded Full Range (DEFR) OCT procedure has been developed, which takes advantage of dispersion mismatch between a sample arm and a reference arm caused by a dispersive material in one arm that results in a broadening of signal peaks in z-space and makes it possible to eliminate complex terms. See B. Hofer, et al., Opt. Express 18, 4898-4919 (2010).
These so-called full range OCT techniques, however, result in useable imaging depths of about 4 mm. The average axial length of the adult human eye is about 24 mm. Thus, ophthalmic clinics must use three or more separate OCT measurements for: 1) imaging the retina, 2) imaging the anterior eye, and 3) measuring the axial eye length.
As a result of these shortcomings, there is a need for improved OCT systems and methods.